arm64: dts: mediatek: x20: Add pinmux support for UART1

Add pinmux support for UART1 on MediatekX20 Development board based
on Mediatek MT6797 SoC.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>